The need for a system of the above type is borne out by the fact that certain material, namely glass fibres, plastic fibres, fine metallic wires and filaments, are normally manufactured and wound on to drums or spools for handling and storing. Moreover once these materials are required to be processed and made into a cable or strand, they have to be payed-off from the stored spools into the process which will transform them into another product.
It is during this unwinding (and also the previous winding) process that care must be takn not to stress the material mechanically as this will either impair the future optical, electrical or mechanical properties of the fibre or, at worst, will break the fibre completely.
It is in this area that a accurate system is required which would perform this duty and thus for example render the fibre being paid-off at constant tension producing a constant characteristic pay-off and enabling the required parameters of the final product to be within specified limits.
As a practical example, consider an optical fibre, which is a glass material of certain refractive index and of the order of 100-200 microns in diameter and which, after manufacture, is to be put into cable form. The eventual cable may contain 5, 10 or 20 of these fibres, each payed-off into an extruder. The extruder then extrudes the material and forms a cable which may be used for data transmission or communications.
For paying-off each individual fibre, a tension control system is required that maintains tension at a few grams, ie 15 or 20 gm, continuously throughout the pay-off process irrespective of acceleration or speed and independently of the weight or size of the spool. The fibre is very fragile, therefore it is very important that the system can maintain this tension without any deviation.
Previous systems in existence include tension control pay-off's employing DC motors such as described in UK Patent No. 1194771.
This prior system was designed essentially for paying-off metallic wire and had tension control requirements in the range 1 to 5 kg. Thus deviation from the range was not critical since the material being payed-off was not fragile or ductile.
Current needs however require a more precise and accurate method of tension control because of the different and varying characteristics of the material to be handled namely that of fragility as mentioned above.
The prior system discussed above is not capable of providing these needs due primarily to the use of a DC motor.
Amongst other disadvantages of employing D.C. machines where sensitivity of operation is essential, is the inclination to cog at low speeds. Moreover hot spots are created in the brushes and commutators and "lurched starting" occurs from stop. The brushes themselves carbonize at zero or low speed, creating high resistance hot spots and ensuing discontinuity rendering the sytem unusable.